Method for removing scale from steel



Patented July 3, 1951 METHOD FOR REMOVING SCALE FROM EEL John A. Lotz, Allison Park, Pa., assignor to The Union Switch & Signal Company, Swissvale, Pa., a corporation of Pennsylvania,

No Drawing. Application December 12, 1946, Serial No. 715,643

2 Claims. 1

My invention relates to an improved method for removing annealing scale or oxide film from certain types of alloy steels containing silicon such, for example, as low silicon content electromagnet steels, tool steels, die steels, etc., as well as carbon and nickel steels.

Heretofore the removal of scale from steels of the type described has frequently necessitated special heat treatments for converting the scale to another chemical form prior to its removal, and has at times resulted in impairment of particular properties possessed by the steels. For example, in the case of electromagnet steels, the scale conversion treatment has frequently resulted in harm to the magnetic properties of the steel.

One object of my present invention is to provide a pickling solution for removing scale from steels of the type described, which solution is relatively inexpensive, is easy to use, and will not harm the special characteristics of the steel.

Another object of my invention is to provide a pickling solution for removing scale from steels of the type described, which solution will not pit the surface of the steels during the process of removing the scale.

Other objects and characteristic features of my invention will become apparent as the description proceeds.

In accordance with my invention, I remove the scale from the steel by subjecting the steel to the action of a pickling bath the active components of which are phosphoric acid and one or more of the halogen acids.

All of the halogen acids will provide satisfactory results but for general pickling, the use of hydriodic acid appears to produce the brightest surface and the brightness decreases progressively with the use of hydrobromic, hydrofluoric, and hydrochloric acid in the order named. However, the difference in brightness is ordinaril not sufiicient to justify the additional expense that would result from the use in the pickling bath of any of the halogen acids other than hydrochloric acid.

The presence of the phosphoric acid in the pickling bath appears to greatly augment the effectiveness of the bath to remove scale, and also to prevent pitting when used in the proper proportions. The relative proportions in the bath of the two acids are not critical, and in general the proportions can be varied over a wide range. However, for the best results to prevent pitting, experiments indicate that the phosphoric acid content of the bath should be apmove the scale decreases.

proximately equivalent in volume to the halogen acid content. For example, in the case of a bath consisting of 10% by volume of HCl, 5% by vol ume of H3PO4 (the acids being of commercial strength, namely 36% H01 and 85% H3PO4) and 85% by volume of water, silicon steels containing up to approximately 5% silicon may be pickled over a long period of time without pitting of the surface. A solution containing 20% HCl by volume and 5% H3PO4 by volume when used on the same steels as the solution first mentioned above will pit the surface of the steel upon long time pickling. A solution of 20% E01, 20% H3PO4 will not produce a pitted steel surface when the steel is pickled for the same length of time. The lowest practical content of hydrochloric acid in the bath appears to be about 10% by volume with a minimum concentration of 5% by volume of phosphoric acid (both of commercial strength) to prevent pitting.

The temperature of the pickling bath may be varied over a wide range from room temperature up to the boiling point, and in general as the temperature increases, the time required to re- A temperature of between 180 and 190 Fahrenheit is recommended. The time required to remove the scale also varies with the type of steel and the concentration of the acids in the pickling bath, and the time required for any particular steel in any particular bath can be readily determined by experiment.

The pickling bath embodying my invention appears to be particularly effective in removing scale from the low silicon content electromagnet steels, but it also produces very satisfactory results on various tool and die steels, as well as carbon and nickel steels.

The bath can be used as an electrolytic pickling bath with current densities as high as it is practical to use. Generally speaking, the higher the current density, the less the time required for scale removal. The minimum practical current density would appear to be about 10 amperes per square foot while for general production a current density of between 50 and 100 amperes per square foot would ordinarily be used depending upon. the amount of scale to be removed and the type of steels from which it is to be removed.

One advantage of a pickling bath embodying my invention is that it eliminates the necessity for an intermediate heat treatment in removing annealing scale from certain forms of electromagnet steels, and thus results in a considerable decrease in the ultimate cost of the steel and the elimination of any danger of harm to the magnetic properties of the steel.

Another advantage of a pickling bath embodying my invention is that it appears to leave a thin phosphoric coating on the steel surface which phosphoric coating appears to prevent corrosion from taking place for some time after the pickling operation has been completed.

Although I have herein shown and described only one pickling solution and two methods for removing scale from steels embodying my invention, it is understood that various changes and modifications may be made therein within the scope of the appended claims without departing from the spirit and scope of my invention.

Having. thus described my invention, what I 2. A method for removing scale from low silicon content steels without pitting which comprises subjecting the steels to the action of a pickling bath consisting of 20% by volume hydrochloric acid (36%), 20% by volume of phosphoric acid (85%) and water.

JOHN A. LOTZ.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name 7 Date 1,195,704 Marino Aug. 22, 1916 FOREIGN PATENTS Number Country Date 19,223 Australia Oct. 1, 1929 of 1929 

1. A METHOD FOR REMOVING SCALE FROM LOW SILICON CONTENT STEELS WITHOUT PITTING WHICH COMPRISES SUBJECTING THE STEELS TO THE ACTION OF A PICKLING BATH CONSISTING OF 10% BY VOLUME HYDROCHLORIC ACID (36%), 5% BY VOLUME OF PHOSPHORIC ACID (85%), AND WATER. 